Field of the Invention
The present invention relates to a lighting device for vehicles, including a housing, which is closed by a transparent cover panel and contains a holographic lamp unit having a light source unit, and including an optical unit containing a holographic element for creating a holographic luminous surface.
Description of the Background Art
A lighting device for vehicles is known from EP 2 354 637 A1, which includes a planar light conductor. Light is coupled in on a narrow side of the planar light conductor. Decoupling elements are disposed on a rear flat side of the planar light conductor in the main radiation direction, so that coupled-in light striking them is reflected in the direction of a front flat side, where it is decoupled. A luminous surface for generating a predefined light distribution is produced hereby.
A disadvantage of conventional lighting device is that the decoupling elements are manufactured by printing or are designed as an eroded or etched structure, so that they are clearly visible to a viewer, particularly even in the non-illuminated state thereof. This is felt to be bothersome. Perceptible optical structures of this type in a luminous surface are avoidable if the lighting device includes a holographic lamp unit having a light source unit, on the one hand, and an optical unit having a holographic element, on the other hand. The holographic element has a structure such that a light beam emitted by the light source unit is diffracted according to a predefined light distribution.
A lighting device for vehicles is known from DE 44 21 306 C2, which corresponds to U.S. Pat. No. 5,455,747, in which the holographic element is mounted on a cover panel of the lighting device. A virtual holographic luminous surface for generating, for example, a direction indicator function runs in the plane of the holographic element. The disadvantage of the known lighting device is that the holographic luminous surface produces only a planar appearance.
Planar lighting elements designed as OLED elements are also known. OLED elements are planar LED light sources for generating a homogeneously illuminated light surface of an arbitrary shape. The OLED technology is highly complex and therefore cost-intensive. In designing a light function using OLED elements, the latter must be structurally accommodated and held in such a way that a holding geometry must always be provided on at least one side of the OLED element, which is visible to a viewer. The appearance of a light function using one or multiple OLED elements may be advantageously improved and replaced by using a virtual holographic luminous surface, since the latter does not require a structural connection but may appear as a floating luminous surface generated in space.